This invention relates to improved gray cast iron and to processes for producing gray cast iron comprising adding microalloying additions of elements with strong affinity for nitrogen, carbon and oxygen to the iron to obtain cast iron that suppresses chemical wear at the cutting edge of cubic boron nitride and silicon nitride cutting tools used during finish machining at high speed and low feed rates.
High speed machining of cast iron has generated great technological interest because it has the potential to offer excellent surface finish under dry machining conditions, increase productivity and decrease cost. In finish turning operations, high speed coupled with low feed rates have been used successfully to achieve excellent surface finish so that a subsequent finish grinding operation is eliminated, resulting in substantial cost savings. Polycrystalline cubic boron nitride (PCBN) tools have been found to be the tools of choice for high speed machining because they exhibit diamond-like structure, high hardness and good thermal conductivity. For example, polycrystalline cubic boron nitride tools are successfully used at high speeds ( greater than 7,200 feet (2,194 m) per minute) and low feed rates (0.006xe2x80x3 (0.15 mm) per revolution) to achieve the excellent surface finish (Ra less than 1 micrometer) as required, for example, in cast iron brake rotors. The problem, however, which has plagued the growth of high speed machining of cast iron, is the chemical wear of the tool occurring at the cutting edge caused by unknown variables in pearlitic iron casting which cause unpredictable tool life and poor surface finish. Attempts have heretofore been made to enhance the strength of the tool by increasing the cubic boron nitride constituent in the tool or by use of other tool additives, but these attempts have not increased the tool life significantly as the tool life is controlled by the mechanism of chemical wear.
Currently, silicon nitride tools and SiAlON, a ceramic tool based on a quaternary system involving Sixe2x80x94Oxe2x80x94Alxe2x80x94N, are most extensively used for machining cast iron. These tools are very cheap in relation to cubic boron nitride tools. If these tools are applied for finish machining cast iron at high speeds and low feed rates, the tool life is again unpredictable for the same reason i.e., chemical wear.
Extensive research has been carried out over the years but a quantitative understanding of the mechanism underlying chemical wear has not been established. Previous studies have focused on foundry practice variables including melting methods, charge materials, inoculation, pouring variables, cooling cycles and cleaning processes. The variables affecting the chemical wear at the cutting edge of the tool, however, have not been isolated. A transition from chemical to abrasive wear on PCBN tools was reported with increased feed rate. Unfortunately, when the feed rate is increased, surface finish on the workpiece deteriorates dramatically. Thus, it is essential to identify the critical variables that control the mechanism of chemical wear occurring at low feed rates in high speed finish machining in order to take corrective measures. Routine metallurgical investigations on castings drawn from batches that machined well in comparison with those that gave poor tool life did not reveal any clues. Statistical approaches based on multi-variants involving machining variables and foundry variables did not resolve the problem as the key parameters relating to solute concentrations of reactive elements, which control the mechanism of chemical wear, were not taken into account.
Accordingly, it is an object of this invention to provide processes which minimize chemical wear at the cutting edge of nitride tools such as cubic boron nitride, silicon nitride, SiAlON, and the like, during high speed machining of gray cast iron in order to achieve consistently good surface finish coupled with prolonged tool life.
It is a further object of this invention to provide improved gray cast irons, which can undergo high speed machining with cubic boron nitride tools while preserving the cutting edge and useful life of such tools.
It is a still further object of this invention to provide improved gray cast irons, which retard chemical wear at the cutting edge of the tool during high speed machining with cutting tools such as cubic boron nitride, silicon nitride or SiAlON.
The foregoing objects as well as other objects and advantages are accomplished by the present invention which comprises a process for producing gray cast iron exhibiting consistently good surface finish with prolonged tool life during finish machining comprising: i) forming a near-eutectic or eutectic melt which upon solidification gives A-type graphite flakes in a pearlitic matrix; ii) adding microalloying elements with strong affinity for nitrogen and carbon to said gray iron melt to combine with dissolved nitrogen and carbon in said iron matrix; iii) adding elements with strong affinity for oxygen to said melt adapted to form a chemically stable, high melting or refractory oxide protective layer on the surface of the tool in contact with said cast iron during finish machining; and iv) inoculating the melt with ferrosilicon based additives, and casting the resulting melt.
This invention is directed to the substantive elimination of the localized chemical wear at the cutting edge of the tool occurring at the high speeds and low feed rates characteristic of finish machining of cast iron. I have now discovered that the oxidation of the cutting tool which occurs at the cutting edge of the cubic boron nitride tool and the silicon nitride tool, respectively, is caused by high temperature due to shear localization in the primary shear zone. The temperature increase is brought about by strength increase in the cast iron due to strain aging. In accordance with my invention, decreased temperature at the cutting edge of the tool can be achieved by preventing strength increase in the cast iron due to strain aging. Strain aging is caused predominantly by interstitial solutes, namely nitrogen and carbon, dissolved in the iron matrix. Scavenging the nitrogen with, for example, titanium as titanium nitride, eliminates dynamic strain aging due to solute nitrogen. By the same token, scavenging the carbon in ferrite with, for example, vanadium as vanadium carbide, eliminates dynamic strain aging due to solute carbon in ferrite. I have discovered that it is possible to keep vanadium in solution in austenite prior to eutectoid transformation because the solubility of vanadium in austenite is increased by carbon in a high carbon austenitic matrix. Chemical wear is caused by oxidation of the cutting edge of the tool due to high temperature. Cubic boron nitride tools are unstable in the presence of oxygen in air and hence readily oxidize forming low melting B2O3. According to the present invention, the tool surface is protected from chemical oxidation by adding reactive elements such as Al to the cast iron matrix, thereby forming a stable refractory Al2O3 layer that protects the cutting edge of the tool by reducing B2O3.
At high cutting speeds, shear localization in the primary shear zone raises the temperature of the cutting edge of the tool. Just like cubic boron nitride tools, silicon nitride tools are unstable in the presence of oxygen in air at high temperature and hence readily oxidize forming silica glass. If a stable refractory oxide can be formed through microalloying addition of elements with strong affinity for oxygen to the gray cast iron workpiece, the tool life can be enhanced substantially. Just as in the case of cubic boron nitride tools, soluble aluminum in the iron matrix in workpiece can reduce silica in-situ to form a more stable refractory oxide enriched in alumina that can preserve the cutting edge of the tool. Thus tool life can be extended by in-situ reaction between a reactive element engineered into the work piece and the unstable oxide formed at the cutting edge of the tool. The reactive element such as aluminum reduces the oxide layer formed on the tool surface and forms a more stable refractory oxide that protects the cutting edge of the tool.